Small internal combustion four cycle engines are known which have a rocker box and a separately formed cylinder head. U.S. Pat. No. 4,601,267 to Kronich, for example, discloses a rocker box base fastened on a cylinder head integrally formed with a cylinder block. Similarly, U.S. Pat. No. 5,058,542 to Grayson et al. discloses a thermoplastic or die cast aluminum inner rocker box cover bolted on a cylinder head.
It is also known to make the cylinder head and rocker box as a one piece, integrally formed cylinder head assembly. The manufacture of such a cylinder head assembly typically involves a complex casting which may require four or more slides to form the intricate air passages and cooling fins by which heat is dissipated from the hottest part of the engine. Because the casting process is so difficult, however, and particularly when it is performed on a very small scale for single cylinder engines, the resulting wall thicknesses are frequently not uniform. Additionally, it is difficult or impossible to cast the rocker box with integral push rod tubes.
Furthermore, the casting process may permit only a single slot on the order of a millimeter wide to be formed on either side of the spark plug and between the cylinder head and the rocker box. Since engine cooling is a function of the air flow through this passage, such a limited air path restricts cooling efficiency. The presence of disuniformly thick walls compounds this problem.
Additionally, non-overhead camshaft small four cycle engines which use cam followers in the valve train typically have a camshaft on which are mounted the camgear and one or more cams, and a follower shaft on which are mounted the cam followers. Conventionally, the follower shaft is mounted to the cylinder block, and the camshaft is mounted to the crankcase. However, this construction introduces variances into the desired operation of the valve train for several reasons. Initially, there is often a variation in the center distance between the shafts because of manufacturing tolerances in the formation of the cylinder block, crankcase and valve train components. Additionally, there is some variance in the width of the gasket which typically separates the cylinder block and the crankcase.
These variances result in a deviation from the optimal functioning of the valves, which can diminish the efficient operation and/or emissions performance of the engine. These variances are magnified in smaller engines such as single cylinder engines used for lawn and garden work. Further reductions in emissions output are known to be obtainable by engine exhaust gas recirculation. However, a simple and cost effective system for exhaust gas recirculation in small engines is not readily available.